Field of the Invention
The present invention relates to a light extraction substrate for an organic light-emitting device, a method of fabricating the same and an organic light-emitting device including the same, and more particularly, to a light extraction substrate for an organic light-emitting device which can overcome a problem that light extraction is caused mainly in a specific wavelength range in a conventional photonic crystal pattern having a periodic structure and can cause light extraction in a wider wavelength range, a method of fabricating the same and an organic light-emitting device including the same.
Description of Related Art
In general, an organic light-emitting diode (OLED) includes an anode, a light-emitting layer and a cathode. When a voltage is applied between the anode and the cathode, holes are injected from the anode into a hole injection layer and then migrate from the hole injection layer through a hole transport layer to the organic light-emitting layer, and electrons are injected from the cathode into an electron injection layer and then migrate from the electron injection layer through an electron transport layer to the light-emitting layer. Holes and electrons that are injected into the light-emitting layer recombine with each other in the light-emitting layer, thereby generating excitons. When such excitons transit from the excited state to the ground state, light is emitted.
Organic light-emitting devices including an OLED are divided into a passive matrix type and an active matrix type depending on the mechanism that drives the N*M number of pixels which are arranged in the shape of a matrix.
In an active matrix type, a pixel electrode which defines a light-emitting area and a unit pixel driving circuit which applies a current or voltage to the pixel electrode are positioned in a unit pixel area. The unit pixel driving circuit has at least two thin-film transistors (TFTs) and one capacitor. Due to this configuration, the unit pixel driving circuit can supply a constant current irrespective of the number of pixels, thereby realizing uniform luminance. The active matrix type organic light-emitting display consumes little power, and thus can be advantageously applied to high definition displays and large displays.
However, as shown in FIG. 5, only about 20% of light generated by an OLED is emitted to the outside and about 80% of the light is lost by a waveguide effect originating from the difference in the refractive index between a glass substrate 10 and an anode 20, and an organic light-emitting layer 30 which includes a hole injection layer and a hole transport layer 31, an emissive layer 32, and an electron transport layer and an electron injection layer 33 and by a total internal reflection originating from the difference in the refractive index between the glass substrate 10 and the air. Specifically, the refractive index of the internal organic light-emitting layer 30 ranges from 1.7 to 1.8, whereas the refractive index of indium tin oxide (ITO) which is generally used for the anode 20 ranges from 1.8 to 1.9. Since the two layers have a very small thickness ranging from 200 to 400 nm and the refractive index of glass used for the glass substrate 10 is about 1.5, a planar waveguide is thereby caused inside the organic light-emitting device. It is calculated that the ratio of the light lost in the internal waveguide mode due to the above-described reason is about 45%. In addition, since the refractive index of the glass substrate 10 is about 1.5 and the refractive index of the ambient air is 1.0, when the light is directed outward from the inside of the glass substrate 10, a ray of the light having an angle of incidence greater than a critical angle is totally reflected and is trapped inside the glass substrate 10. Since the ratio of the trapped light is up to about 35%, only about 20% of the generated light is emitted to the outside.
In order to improve the luminous efficiency of an organic light-emitting device, a variety of conventional light extraction approaches was proposed. One of these light extraction approaches employs a photonic crystal structure that has a periodic pattern to extract light, the periodic pattern being formed at the front side of the organic light-emitting device through which light from the OLED is emitted. The size and period of the photonic crystal structure depend on a wavelength, and thus improvement in light extraction is limited to a specific wavelength range. The photonic crystal structure causes a phenomenon in which the peak of one wavelength in a specific wavelength range is higher than that of other wavelengths or the wavelength peak is shifted. Accordingly, the conventional photonic crystal structure is not applicable to white organic light-emitting devices for lighting application, the uniform luminous intensity of which must be obtained in a wide wavelength range.
The information disclosed in the Background of the Invention section is provided only for better understanding of the background of the invention and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.